The present disclosure relates to a method of providing a fixture for a ceramic article in a machine tool such that the ceramic article may be machined to a required shape. The present disclosure also relates to a method of machining a ceramic article, e.g. machining a ceramic block into a ceramic casting core. The present disclosure also relates to a method of investment casting using a ceramic article, e.g. ceramic casting core.
A fixture's role within manufacturing is to allow the repeatable deterministic positioning of an article, or a component, relative to a particular manufacturing process or machining tool. A fixture ensures that the article, or component, is positioned in such a manner that it is held within the working volume of the manufacturing process, or machining tool, relative to a working datum. Accurate positioning of an article, or component, relative to the manufacturing process, or machining tool, is vital for the repeatable production of accurate parts, especially in high volume manufacturing processes.
To mathematically define the position of a ridged body in three-dimensional space, six parameters must be defined, the position in the X-direction, position in the Y-direction, the position in the Z-direction, rotation about the X-axis, rotation about the Y-axis and rotation about the Z-axis. These six spatial parameters are described as the degrees of freedom of the rigid body. A fixture's role is to deterministically set the position of the article, or component, by controlling the degrees of freedom of the article or component, e.g. preventing movement of the article or component. Once the position of an article, or component, has been determined relative to a datum coordinate frame, the features of the article, or component, may be referenced repetitively.
When fixturing a rigid prismatic article, or component, 1 its position in three dimensional (3D) space is most commonly determined using a 3-2-1 location strategy. The articles degrees of freedom are restrained first through three points 2 of contact in a plane, two points 3 of contact in a line and finally one point 4 of contact, which provide complete location of the article 1, see FIG. 1.
The contact points of the article which restrain movement of the article are critical features that must be tightly controlled to achieve a high level of repeatability in the location strategy. Otherwise, as each article varies slightly, the deformations will propagate to successive manufacturing processes, or machining steps, based upon location error and this will result in a large amount of variability from article to article. This is not a significant problem for most metallic articles, or other hard material articles, as these points of contact can be consistently maintained. However, for brittle material articles with low surface wear resistance, for example sintered ceramic articles, this may be a significant problem. When a brittle material article is placed against these contact points, the contact points may produce wear of the brittle material article and this may alter the position of the brittle material article upon relocation. Furthermore, the contact points may also chip, or crack, the surface of brittle material articles at these critical locations. The damage to the brittle material articles may increase when the brittle material articles are subjected to restraint and manufacturing processes. Dimensional variability caused by the sintering of ceramics may also compromise the location repeatability of ceramic articles, or ceramic components, making it difficult to utilise a stable reference datum.
Accordingly the present disclosure seeks to provide a method of providing a fixture for a ceramic article which reduces, or overcomes, the above mentioned problem.